The Hammers were taken to a replay after drawing 2-2 with the Premier League leaders at Upton Park, when Robin van Persie’s stunning stoppage time strike cancelled out James Collins’ headed double.

Centre back Collins has since joined fellow defenders Joey O’Brien and George McCartney on the sidelines after suffering a hamstring tear in Saturday’s 3-0 defeat at Sunderland, leaving Allardyce with an injury crisis on his hands.

But the Hammers boss insists his depleted side will have a chance of an upset at Old Trafford if they remain patient and snatch the rare opportunities that come their way.

“A struggling team will do more desperate things and you don’t want to get yourself into a struggling situation against Manchester United,” he said.

“You try to focus on your team keeping their composure, keeping their cool and not chasing the game.

“Trying to snatch it, that’s the mentality - waiting for the opportunity that doesn’t come very often at Old Trafford.

“If we get that right we might have a chance, if they are not quite as good as normal.”

West Ham were beaten 1-0 at Old Trafford in the league back in November, when a deflected first minute strike from Van Persie proved decisive.

But Allardyce was impressed with how his side stifled the Red Devils that day after the early setback and says they must look to frustrate the hosts again.

“We did very well in the league after recovering from a tricky start, stifling Manchester United’s huge amount of flair on the front line,” he said.